In the gemara we see an example of migu, where the woman could lie and claim to be mukatz etz. Because she doesn't we believe her somewhat weak claim.
I have three questions:
1)Broadly is migu always based on a distinction between truth and lie, that if the person could have lied credibly we believe they are telling the truth, since it's now clear they're not lying? Or is it also if they could have made their truthful argument better, we give them credit "as if" they'd made the best argument?
2) isn't there a risk that if people know this halacha they'll abuse it by making weak truthful claims vs an outright lie? Is the "drisha and chakira" different in migu?
3) in general, how does this square with the concept of swearing an oath in order to not pay? If the person is a thief and a liar is he really going to be so worried about swearing a false oath? And even if this was the case in the times of the mishna, is it true in modern days...do batei din in 2021 still allow people to take an oath and be released from a need to pay?
1)
a) It seems obvious to me that if the alternative argument was also true, then the arguer is certainly believed. I do not think that a proof is required for this, but I will cite the Rashbam to Bava Basra 32b (DH Amai) who writes:
"For every 'Mah Li l'Shaker' in the Gemara, both arguments could be true -- the one that he argued, and the one that he could have said but did not say, so we say 'Migo.'"
b) Also, I found that Tosfos (9b, DH Lo) asks that we should believe her with a Migo that she could have said "Mukas Etz Ani." The Derush v'Chidush (p. 70, end of second column) answers in the name of the Rosh that Mukas Etz is unusual. The Derush v'Chidush explains that even though the argument of "I was Ne'enas after Erusin" is believed, according to Raban Gamliel, with a Migo of Mukas Etz, this is because it is a Migo from a "Ta'anah Garu'ah," a weak argument, to another Ta'anah Garu'ah. We learn that we say a "Migo mi'Ta'anah Garu'ah to Ta'anah Garu'ah."
We see from this that the arguments do not have to be strong ones. It is sufficient that they be plausible ones.
2) Answer to question #2:
a) I found, bs'd, in the Teshuvos Rivash (end of #392, DH Aval Ro'eh; the Rivash lived from 1326-1408) that he writes that "the masses in these areas are expert at Migo arguments ('Melumadim b'Ta'anas Migo') and they have made it into a habit to lie and cheat each other. Therefore, the judge must perform a thorough Derishah and Chakirah."
b) A different (possibly opposite) answer to question #2 is as follows:
I found that the Sefer Urim v'Tumim (by Rav Yonasan Eibeshitz) on the Shulchan Aruch, Choshen Mishpat, end of Siman 82, cites the "Short Rules of Migo," written by the Sefer Keneses ha'Gedolah.
In #28 he writes that one cannot believe an Am ha'Aretz with a Migo that he could have made a different argument, because it is possible that he did not know the alternative argument since it is not well-known by everyone.
The source for this is the Rosh in Kesuvos (9:28, at the end). The Rosh discusses a Migo and writes that one cannot rely on it because not everyone knows the Halachah. This seems to be the opposite to the way we have been understanding up to now, since the Rosh learns that a Migo works only if one knows the alternative. I do not know yet how this fits with the Rivash I cited above. It seems that there is a dispute here since the Shach (Choshen Mishpat 66:56) disagrees with the Rosh.
c) Here is the opinion of the Shach, cited above. The Shach writes that what the Rosh writes -- that a Migo is not effective if the person making it does not know the Halachah -- is illogical. He writes that we find many cases of a Migo which would require a lot of in-depth study in order to understand the Migo, and that the whole of Shas and Poskim is full of such cases of Migo. We do not say that possibly the person making the argument does not understand the Din.
I want to suggest an explanation for the dispute between the Rosh and the Shach, according to the famous Chakirah concerning a Migo: Do we rely on a Migo because it proves that the person saying the Migo is telling the truth with his present claim and has "Ne'emanus," believability, or is it because of "Ko'ach ha'Ta'anah" -- the Migo gives him a power of argument even though it does not actually improve the actual credibiltiy of the argument he is making?
This question is cited in Kovetz Shi'urim (2:3:12), and in 2:3:15 the Kovetz Shi'urim writes that according to the Rosh the Migo is relied upon because "Anan Sehadei" -- "we are witnesses" that the person who makes it is not lying. I assert that this fits with the Rosh that I cited earlier, that if a person does not understand the Migo, it cannot be relied upon.
In contrast, the Shach learns that Migo works through "Ko'ach ha'Ta'anah." For example, when she says there was Ones after the Erusin, she is believed because the power of the argument of "Mukas Etz" is transferred by the Migo onto the argument of "mi'she'Erastani Ne'enasti." She does not necessarily need to understand the logic of this argument.
Kol Tuv,
Dovid Bloom